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Small Cabin Forum / Off-Grid Living / My head is melting about Batteries!!
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# Posted: 6 May 2019 16:09

there are cheaper options than renogy out there,but the basis of your math is right. Its the fact that you will get 5 or more times the lifespan out of the lithium that make them cost effective over the long term.

If you deal with below freezing temps, that brings in another negative for lithium, but it can be dealt with fairly easily.

# Posted: 6 May 2019 18:01

Don't feel bad about solar information being overwhelming. It took me over two years of research and self teaching to start to understand it. I am no pro for sure but I received great help from this and other sites. My system is small but I have the camp wired for 110 with plugs and light switches. Suggestion, wire the cabin during the building phase just like a house even if you delay any power decisions. My system works great. Also, my propane fridge works fine but if your wife is happy with the coolers - go with that for now but plan ahead for future upgrades.

# Posted: 6 May 2019 19:11 - Edited by: NorthRick

Quoting: zorro
So would I actually need 2 of the lithium to get me to 4080Wh, still 1000Wh less than the T105’s?I know they are longer lasting, less maintenance, etc, but if the above is correct, extremely pricey?

Yep, they are pricey. Proponents for lithium like to say that since they are projected to last much longer than lead batteries that they aren't as expensive long term. However, they haven't been in use long enough real world to know for sure. No one who says they will last 20 years has used them for 20 years. Lead batteries have been around for a 100 years and their capacities and lifespans are well known.

One of the big advantages for lithium is that you can leave them in a partially discharged state for extended period without substantial degradation. Lead batteries need to be stored at 100% state of charge or they will lose capacity and lifespan. This difference is a big deal if you are cycling your batteries daily and they rarely are at 100% SOC.

In your case, you will be gone more than you are there and if you have an operating solar system they will spend much of their time sitting at 100% SOC. That's how it is with the system at my cabin. They are Costco golf cart batteries going on 9 years.

I am building a new (bigger) cabin and unless lithium prices really come down before I'm done, I will be setting up a flooded lead acid battery bank.

# Posted: 13 May 2019 15:00 - Edited by: zorro

So just following up on this a little and a few ore thoughts

The cooler idea for the fridge has been shot down by the “boss”………………so need to rethink that again

So the thoughts are;

2 x 300W solar panels
8 x Deka GC15 – 6v x 230a = 11040 Ah x 60% = 6624Ah
1000W sine wave converter
Generator - as back up/for the well (4/5000W generator)

So looking at;

Heating – wood stove
Well – generator
Power – solar (with generator back up)
Fridge – either propane or……………could a small electric fridge run off the solar system

Fridge would be this; rtment-Black/50288017

115v x 1.6a – 184Ah

Would that be a possibility or too much of a drain on the solar system?

Or would that need more panels/batteries to operate comfortably?

Idea would be to have it running most of the summer if the above system would cope with it?

Nate R
# Posted: 14 May 2019 10:50

The only issue I see here is that you are under paneled for that size battery bank if you're doing flooded lead acid.

See the link above. You generally want to be able to charge at 10-13% of the C20 AH rating for various reasons when in bulk. So, you're looking at 230 AH batteries, you want to be able to send at least 23 amps to them in peak sun to keep things mixed well, etc. Need at least 1100W in panels to do that, plus charging, panel losses, etc. I'd go for 4 300 watt panels, or just accept the batteries might not last as long, OR charge with the generator sometimes, with a 30 amp charger. (Iota makes a 48V, 30 A 240V AC charger.)

Fridge energy guide is 236 KWH/year, or 647wh/day. (236/365). Add in inversion and charging losses, adn your'e looking at sucking at least 750-800wh/day in the fridge. I think you'll be OK, in my opinion.

# Posted: 14 May 2019 11:31

Thanks for the info and suggestions

I never realized I was under on the panels – they are not too big a cost, so could go to 4 x 300W

4 x 300W/24V Mono Panels

So the other items appear to be okay for the 4 panels and 8 battereis, in terms of the;

1200W Pure Sime Wave Inverter

40A MPPT Charge Controller (Max 100V, 520W/1040W Input for Lead-Acid) – or should that go up to 50A?

And thanks for the info on the power use of the fridge – that was also helpful

Nate R
# Posted: 14 May 2019 13:27 - Edited by: Nate R

Watch your panel voltage vs the charge controller. Panel voltage goes up in the cold, so you either need to have enough headroom for that, or you need to wire the panels in parallel, which means more amps and thicker wire. Just have to wire it right and account for it.

With those panels, you'd have 2 sets of 2 in parallel, connected in series. That would give you about 64 volts and 19.5 Amps in full sun. (Or, when it's cold, 80-90 volts.)

So now, you need to be careful with voltage drop with your wires. 10 AWG wire between the panels and controller would be good for 50 feet when wired that way. 7&voltage=64&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=19.5&x=70 &y=21

Many say 2-3% max, but some say 5% voltage drop is OK. You have to balance the loss in power vs wire cost, etc.

Otherwise, to my eye, looking good. 40 amps is fine, you don't really want to be able to charge much higher than 13% of the C/20 AH rating for flooded, according to Trojan. Many charge controllers can limit charge output amps, and you won't likely spend a ton of time with the panels at max output.

# Posted: 14 May 2019 14:45 - Edited by: zorro

Thanks for the heads up on the wiring and distances – hopefully my panels should be no further than about 20-30 feet from the batteries and controllers

I am a little confused on the "2 sets of 2 in parallel, connected in series" suggestion

(I presume the reason I would not do all 4 in series is that would give me 96V/9.8A………..running close to the 100V max of the Charge Controller?)

I will have 4 panels, each at 24V/9.8amps

Are you suggestion that I do 2 x 2 in parallel and then connect those 2 banks in series?

So I would have…………..

1st Bank - 24V/9.8A + 24V/9.8A = 24V/19.6A
2nd Bank - 24V/9.8A + 24V/9.8A = 24V/19.6A

But cant see how that comes out to 64V/19.5A

That is where I am getting a little lost if you can explain a little more

I really appreciate the help here as this is a new world to me and trying to read as much as I can about it all

# Posted: 14 May 2019 16:52


You want to put 2 of the 4 panels in series and do that with the other 2. Then connect those two strings in parallel.

The panels you listed are 24 volts nominal. That's not what they operate at (like a 2x4 not being 2" by 4"). The spec sheet for your panels lists the following:

Open Circuit Voltage (Voc): 39.82V
Short Circuit Current (Isc): 9.78 A

These values are higher than the operating values (which is what it looks like Nate used for the voltage) but you want to use them when figuring out what charge controller to use.

With 2S2P you could see 39.82v x 2 = 79.64v
and 9.78a x 2 = 19.56a

However, this is at 25C or 77F. Solar panels perform better in colder weather so these values can go higher.

The charge controller you linked to can handle 92v at 25C (even though it otherwise says max 100V) coming from the panel to the controller. I think this cuts it a bit close and would look for a different controller that can handle more volts.

# Posted: 15 May 2019 14:38

Perfect - think I have got it now!

Thanks guys


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